Chore: Document Conversation APIs in swagger (#552)

* Added the documentation for the conversation APIs - 8 endpoints
* Added descriptions for the existing endpoints
* Added new resource models and updated some parts of the contact API

swagger/definitions/request/conversation/create_message.yml

@@ -0,0 +1,10 @@
+type: object
+properties:
+  conversation_id:
+    type: number
+  message:
+    type: string
+  private:
+    type: boolean
+  fb_id:
+    type: number